Method of improving the removal of investment casting shells

ABSTRACT

The removal of an investment casting shell surrounding a metallic part is improved by adding a salt of alkali or alkaline earth metal to at least one of the layers of the shell.

FIELD OF THE INVENTION

This invention relates generally to investment casting and, moreparticularly, to a method of improving the removal of investment castingshells.

BACKGROUND OF THE INVENTION

Investment casting, which has also been called lost wax, lost patternand precision casting, is used to produce high quality metal articlesthat meet relatively close dimensional tolerances. Typically, aninvestment casting is made by first constructing a thin-walled ceramicmold, known as an investment casting shell, into which a molten metalcan be introduced.

Shells are usually constructed by first making a facsimile or patternfrom a meltable substrate of the metal object to be made by investmentcasting. Suitable meltable substrates may include, for example, wax,polystyrene or plastic.

Next, a ceramic shell is formed around the pattern. This may beaccomplished by dipping the pattern into a slurry containing a mixtureof liquid refractory binders such as colloidal silica or ethyl silicate,plus a refractory powder such as quartz, fused silica, zircon, aluminaor aluminosilicate and then sieving dry refractory grains onto thefreshly dipped pattern. The most commonly used dry refractory grainsinclude quartz, fused silica, zircon, alumina and aluminosilicate.

The steps of dipping the pattern into a refractory slurry and thensieving onto the freshly dipped pattern dry refractory grains may berepeated until the desired thickness of the shell is obtained. However,it is preferable if each coat of slurry and refractory grains isair-dried before subsequent coats are applied.

The shells are built up to a thickness in the range of about ⅛ to about½ of an inch (from about 0.31 to about 1.27 cm). After the final dippingand sieving, the shell is thoroughly air-dried. The shells made by thisprocedure have been called “stuccoed” shells because of the texture ofthe shell's surface.

The shell is then heated to at least the melting point of the meltablesubstrate. In this step, the pattern is melted away leaving only theshell and any residual meltable substrate. The shell is then heated to atemperature high enough to vaporize any residual meltable substrate fromthe shell. Usually before the shell has cooled from this hightemperature heating, the shell is filled with molten metal. Variousmethods have been used to introduce molten metal into shells includinggravity, pressure, vacuum and centrifugal methods. When the molten metalin the casting mold has solidified and cooled sufficiently, the castingmay be removed from the shell.

Investment casting molds must withstand significant mechanical anddrying stresses during their manufacture. Ceramic shells are designedhaving high green (air dried) strength to prevent damage during theshell building process. Once the desired mold thickness is achieved, itis dewaxed and preheated to approximately 1800° F. At this point, it isremoved from the high temperature furnace and immediately filled withliquid (molten) metal. If the mold deforms while the metal issolidifying (or in a plastic state), the casting dimensions will likelybe out of specification. To prevent high temperature deformation, moldsare designed to have substantial hot strength. Once the casting issolidified and cooled, low fired strength is desired to facilitate theknock out or removal of the ceramic mold from the metal casting.

Most investment casting molds contain significant quantities of silica.The silica usually starts as an amorphous (vitreous) material. Fusedsilicas and aluminosilicates are the most common mold materials. Whenexposed to temperatures above approximately 1800° F., amorphous silicadevitrifies (crystallizes) forming beta cristobalite. Cristobalite haslow (alpha) and high (beta) temperature forms. The beta form has aspecific gravity very close to that of amorphous silica so molddimensions remain constant and stresses associated with the phasetransformation are minimal. Upon cooling, beta cristobalite transformsto the alpha form. This phase transformation is accompanied by anapproximate 4% volume change that creates numerous cracks in the shell,thereby facilitating mold removal. Cristobalite reduces the firedstrength of silica containing investment casting molds.

Although investment casting has been known and used for thousands ofyears, the investment casting market continues to grow as the demand formore intricate and complicated parts increase. Because of the greatdemand for high-quality, precision castings, there continuously remainsa need to develop new ways to make investment casting shells moreefficiently, cost-effective and defect-free. For instance, if shellstrength was maintained to the point of metal solidification, followedby a reduction in strength as the shell cools, improvements inproductivity could be realized through improved knock out (shellremoval). This is particularly desirable for non-ferrous alloys, e.g.alloys of aluminum, copper and magnesium, because their melting andpouring temperatures are insufficient to promote cristobalite formationand easy knock out.

The knock out is especially difficult when the part presents a blindhole or a small cavity in which the ceramic is under compression. Thecompression occurs during the cooling of the metal parts, which ingeneral have a higher coefficient of thermal expansion (CTE) than theceramic shells. This effect is especially accentuated in non-ferrouscastings because of the high CTE of this metal (>18 10⁻⁶ m/m).

Non-ferrous castings produced by investment casters are rather fragile,so they are cleaned by water or sand blasting, compared with theaggressive shot blast and vibratory cleaning for steel and hightemperature alloy castings. Residual ceramic on steel castings isdissolved away using concentrated acids and bases or molten salt baths.Chemical incompatibility excludes their use on aluminum and magnesium.If a binder was developed having low fired strength and associated easyknock out properties upon exposure to temperatures at or below 1800° F.,aluminum casting cleanup could be greatly improved.

Accordingly, it would be desirable to provide an improved method ofremoving an investment casting shell surrounding a metallic part.

SUMMARY OF THE INVENTION

The method of the invention calls for adding a salt of alkali oralkaline earth metal to at least one of the layers of an investmentcasting shell. The addition of a salt of alkali or alkaline earth metaleffectively improves the removal of the investment casting shellsurrounding a metallic part.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a method of improving the removalof an investment casting shell surrounding a metallic part. Inaccordance with the invention, a salt of alkali or alkaline earth metalis added to at least one layer of the investment casting shell.

The salt of alkali or alkaline earth metals which may be used in thepractice of the invention include calcium carbonate, calcium sulfate,calcium magnesium carbonate, magnesium carbonate, magnesium sulfate,strontium carbonate and mixtures thereof The preferred salt of alkali oralkaline earth metal for use in improving the removal of an investmentcasting shell from a metallic part is calcium carbonate.

The salt of alkali or alkaline earth metal can be added to at least oneof the layers of the investment casting shell by any conventional methodgenerally known to those skilled in the art. In a preferred embodiment,the salt of alkali or alkaline earth metal is added to at least onelayer of the refractory stucco. However, in the practice of the presentinvention, the salt of alkali or alkaline earth metal may alternativelybe added to at least one layer of the refractory slurry or to at leastone layer of both the refractory slurry and the refractory stucco.

The salt of alkali or alkaline earth metal is used at a concentrationthat will effectively improve the removal of an investment casting shellsurrounding a metallic part. It is preferred that the amount of salt ofalkali or alkaline earth metal which is added to at least one layer ofthe shell be in the range of about 1 to about 30% by weight of theshell. More preferably, the amount of salt of alkali or alkaline earthmetal is from about 5 to about 25%, with about 8 to about 20% being mostpreferred.

The present inventor has discovered that adding a salt of alkali oralkaline earth metal to at least one layer of an investment castingshell effectively improves the removal of the shell surrounding themetallic part.

EXAMPLES

The following examples are intended to be illustrative of the presentinvention and to teach one of ordinary skill how to make and use theinvention. These examples are not intended to limit the invention or itsprotection in any way.

Example 1 CaCO₃ Mixed in Refractory Slurry

Slurries were prepared using the following formulas: TABLE 1 SlurryIngredients Concentrations (ratios) Colloidal silica¹ 1920 g Deionizedwater  386 g Latrix ® 6305 polymer²  138 g Nalcast ® P1 (−200 mesh)fused silica³ 1200 g Nalcast ® P2 (−120 mesh) fused silica⁴ 3600 gNalco ® 8815 anionic wetting agent⁵   1.0 g Dow Corning ® Y-30 antifoam⁶  4.0 g CaCO₃ fine Marblemite ®⁷ 0.0 g (0%), 288 g (6%), 576 (12%), 864g (18%), 1200 g (25%) CaCO₃ coarse 40-200⁸ 480 g (10%), 960 (20%), 1680g (30%)¹Nalcoag ® 1130 (8 nanometer, sodium stabilized) (available from OndeoNalco Company)²Styrene butadiene latex at 10% based on diluted colloidal silica(available from Ondeo Nalco Company)³Available from Ondeo Nalco Company⁴Available from Ondeo Nalco Company⁵70% sodium dioctyl sulfosuccinate (available from Ondeo Nalco Company)⁶30% silicone emulsion (available from Dow Corning Corporation ofMidland, Michigan)⁷Cultured marble calcium carbonate (available from Imerys Corporation ofRoswell, Georgia)⁸Screened grade calcium carbonate (available from Imerys Corporation ofRoswell, Georgia)

After seventy-two hours of mixing, the viscosities of the slurries weremeasured and adjusted using a number four Zahn cup. The viscositiesranged from 14-18 seconds. Minor binder additions (colloidalsilica+water+polymer) were made to obtain the desired rheology. Onceadjusted, the slurries were ready for dipping.

Wax patterns were cleaned and etched using Nalco® 6270 pattern cleaner(available from Ondeo Nalco Company) followed by a water rinse. Wax barswere dipped into each slurry followed by Nalcast® S1 and S2 fused silicastucco (available from Ondeo Nalco Company) (applied by the rainfallmethod). Dry times started at 1.5 hours and progressed up to 3.5 hoursas coats were added. The final shells had two coats with Nalcast® S1stucco, three coats with Nalcast® S2 stucco plus one seal coat (nostucco). All coats were dried at 73-75° F., 35-45% relative humidity andair flows of 200-300 feet per minute. After a twenty-four hour finaldry, the shells were placed into a desiccator for an additionaltwenty-four hours prior to testing.

Several shell properties were evaluated using modulus of rupture (MoR)bars prepared from the experimental slurries. The bars were broken witha three point bending fixture on an ATS universal test machine(available from Applied Test Systems, Inc. of Butler, Pa.). The analogoutput (voltage) was fed into a personal computer containing ananalog-to-digital conversion board and data acquisition software. Thedata was stored as a load versus time, or load versus displacement plot.Calculations and analyses were performed using data acquisition softwareor spreadsheet programs. The following physical properties weredetermined for the MoR specimens:

Fracture Load

The fracture load is the maximum load that the test specimen is capableof supporting. The higher the load, the stronger the test specimen. Itis affected by the shell thickness, slurry and shell composition. Thisproperty is important for predicting shell cracking and related castingdefects. The fracture load is measured and recorded for test specimensin the green (air dried), fired (held at 1800° F. for one hour andcooled to room temperature) and hot (held at 1800° F. for one hour andbroken at temperature) condition. Results are normalized and expressedas an Adjusted Fracture Load (AFL). The AFL is simply the fracture loaddivided by the specimen width for a two inch test span.

Shell Thickness

Shell thickness is influenced by slurry and shell composition, combinedwith the shell building process. Thickness fluctuations are indicativeof process instability. Non-uniform shell thickness creates stresseswithin the shell during drying, dewaxing, preheating and pouring. Severecases lead to mold failure. The mold surrounds and insulates the coolingmetal. Changes in thickness can affect casting microstructure,shrinkage, fill and solidification rates.

Modulus of Rupture

A flat ceramic plate is prepared using a rectangular wax bar as thepattern. Typical dimensions are 1×8×¼ inches. The bar is invested usingthe desired shell system. After drying, the edges are removed with abelt sander. The two remaining plates are separated from the wax,yielding two test specimens. The specimens are broken using a threepoint loading apparatus on an ATS universal test machine. MoRs arecalculated for bars in the green, fired and hot conditions.${MoR} = \frac{3{PL}}{2{bh}^{2}}$where P=Fracture load in pounds

-   -   L=Specimen length in inches (distance between supports)    -   b=Specimen width at point of failure in inches    -   h=Specimen thickness at point of failure in inches

The MoR is a fracture stress. It is influenced by fracture load andspecimen dimensions. Shell thickness is of particular importance sincethe stress is inversely proportional to this value squared. The unevennature of the shell surface makes this dimension difficult to accuratelymeasure, resulting in large standard deviations. This deficiency isovercome by breaking and measuring a sufficient number of testspecimens.

Bending or Deflection

The test specimen bends as the load is applied. The maximum deflectionis recorded as the specimen breaks. Bending increases with flexibilityand polymer concentration. A flexible shell is capable of withstandingthe expansion and contraction of a wax pattern during the shell buildingprocess. Bending is measured for bars in the green condition.

Fracture Index

The fracture index is a measure of the work or energy required to breaka shell in the green condition. It is indicative of shell “toughness”,i.e., the higher the index, the tougher the material. For example, apolypropylene bottle is “tougher” than a glass bottle and therefore hasa higher fracture index. The index is an indicator of crack resistance.High index shells require more energy to break them than low indexsystems.

The fracture index is influenced by slurry and shell composition.Polymer additives increase the index. Soft polymers produce higher indexshells than stiff ones. The index is proportional to shell flexibility.A shell that is capable of yielding absorbs more energy than a rigid,brittle one.

The fracture index is determined by integrating the area beneath theload/displacement curve for a MoR test specimen. The index measures(force)×(distance) when monitoring displacement or (force)×(time) whenmonitoring load time. To convert from (force)×(time) to(force)×(distance), the loading rate is used. Test results arenormalized by simply dividing the index value by the specimen width fora two inch test span.

Modulus of Elasticity

The modulus of elasticity, also known as Young modulus, MoE or elasticmodulus is a proportionality constant obtained from the stress-straincurve. It can be calculated from MoR (bending) analysis. The modulus ofelasticity is a measure of stiffness of the material. A stiff, rigidmaterial will display a strain/stress curve with a steep slope and highMoE. A soft, flexible material will exhibit a strain/stress curve with aflatter slope and low MoE. The modulus of elasticity is independent ofsample dimension. It provides an accurate mean for comparing differentshell systems.

As shown below in Table 2, the fired strength decreased with the amountof fine or coarse CaCO₃ in the shell. The best system was achieved withthe addition of 10-20% coarse CaCO₃, which showed approximately a 30%decrease in fired MoR with a minimal effect on the green properties ofthe shell. TABLE 2 Summary green data MoR % CaCO₃ (psi) AFL (lbs) MoE(kpsi) F Index Bending (mils) Fine  0% 556.73 9.02 381 26.43 4.77  6%448.06 8.20 231 28.72 5.46 12% 406.47 7.96 228 28.11 5.03 18% 353.207.51 185 26.25 5.29 30% 261.82 6.26 115 23.59 5.75 Coarse 10% 588.838.29 346 31.26 5.44 20% 482.67 9.00 253 31.57 5.37 30% 385.94 9.46 18634.70 5.33 Summary fired data MoR (psi) AFL (lbs) % CaCO₃ Fine  0%1016.82 15.33  6% 804.23 14.82 12% 651.27 12.59 18% 504.93 11.54 25%397.56 9.05 % CaCO₃ Coarse 10% 752.87 13.91 20% 738.12 15.11 30% 530.5311.87

Example 2 CaCO₃ Added as Discrete Dry Refractory (Stucco) Coats

Slurries were prepared using the following formulas: TABLE 3 SlurryIngredients Concentrations (ratios) Colloidal silica  1920 g Deionizedwater   401 g TX-11280 polymer   138 g Nalcast ® P1 (−200 mesh) fusedsilica  1200 g Nalcast ® P2 (−120 mesh) fused silica  3600 g Nalco ®8815 anionic wetting agent  2.2 g Dow Corning ® Y-30 antifoam  4.0 gAlternative layers of CaCO₃ and SiO₂ stucco were used during the shellpreparation sequence as follows: Stucco Stucco Stucco Coat Coat CoatStucco Coat Stucco Coat Formulation # #1 #2 #3 #4 #5 1 S1 S1 S2 S2 S2 2S1 S1 CaCO₃ ⁹ S2 S2 3 S1 S1 CaCO₃ 50/50 S2 + S2 CaCO₃ 4 S1 S1 CaCO₃CaCO₃ S2 5 S1 S1 CaCO₃ CaCO₃ CaCO₃The shell test methods were also the same.⁹Screened grade calcium carbonate (available from Imerys Corporation ofRoswell, Georgia)

As shown below in Table 4, the addition of the CaCO₃ in the stucco layerdecreased fired shell strength without compromising green strength. Thebest results were obtained with 1 CaCO₃ stucco layer showing a 35% firedstrength decrease with a minimal effect on hot strength. The green, hotand fired MoR results for shells with and without CaCO₃ stucco coat wereas follows: TABLE 4 Summary green data MoR Bending Formulation # (psi)AFL (lbs) MoE (kpsi) F Index (mils) 1 434.51 9.38 175 39.17 6.42 2470.52 7.11 238 28.64 6.30 3 504.09 5.95 294 23.10 6.53 4 473.57 5.30284 22.07 6.34 5 455.91 4.40 292 18.92 6.18 Summary hot and fired dataHot Fired Formulation # MoR (psi) MoR (psi) Hot AFL (lbs) Fired AFL(lbs) 1 1204.13 707.82 19.80 14.36 2 1023.13 465.12 13.14 7.40 3 619.81377.00 9.05 4.96 4 700.56 254.34 7.57 3.09 5 518.13 131.40 4.65 1.38

Example 3 CaCO₃ Mixed with Dry Refractory Stucco

Slurries were prepared using the following formulas: TABLE 5 SlurryIngredients Concentrations (ratios) Colloidal silica  1920 g Deionizedwater   401 g TX-11280 polymer   138 g Nalcast ® P1 (−200 mesh) fusedsilica  1200 g Nalcast ® P2 (−120 mesh) fused silica  3600 g Nalco ®8815 anionic wetting agent  2.2 g Dow Corning ® Y-30 antifoam  4.0 gBlends of SiO₂ and CaCO₃ were used as stucco coats during the shellpreparation sequence as follows: Stucco Stucco Stucco Formu- Coat CoatCoat Stucco Coat Stucco Coat lation # #1 #2 #3 #4 #5 1 S1 S1 S2 S2 S2 2S1 S1 S2 + 5% S2 + 5% CaCO₃ S2 + 5% CaCO₃ ¹⁰ CaCO₃ 3 S1 S1 S2 + 10% S2 +10% CaCO₃ S2 + 10% CaCO₃ CaCO₃ 4 S1 S1 S2 + 20% S2 + 20% CaCO₃ S2 + 20%CaCO₃ CaCO₃ 5 S1 S1 S2 + 30% S2 + 30% CaCO₃ S2 + 30% CaCO₃ CaCO₃¹⁰Screened grade calcium carbonate (available from Imerys Corporation ofRoswell, Georgia)

An erosion test was added to fully characterize the benefit of the CaCO₃addition on the knock out properties of the shells. The new techniquesimply consisted of a simulation of the sand blasting process used inthe industry. Shell coupons of dimensions: L, W=0.75″ to 1.5″ and 0.4″thick max were placed in a fixture located on one of the walls of theblasting cabinet. A spring blade held the coupon against a stainlesssteel mask with a 0.5 x 0.5″ window. This mask ensured that identicalsurface area was exposed to the blast from one sample to the next and ahomogenous blasting media flowed across the surface. The thickness ofthe coupon was measured prior to the test. The coupon was then exposedto the blasting media until it was perforated. The time to perforationwas measured and an erosion speed was calculated for each sample. Thetest was repeated with a representative number of coupons to allow anaccurate determination of the erosion speed.

As shown below in Table 6,,the addition of the CaCO₃ blended with SiO₂in the stucco layer decreased fired shell strength without compromisinggreen strength. The best results were obtained with 10-20% CaCO₃addition to the stucco layer showing a 20 to 40% fired strength decreasewith a minimal effect on hot strength. The erosion speed data show thata five fold increase in erosion speed was achieved after 24 and 48 hourscooling shells containing 8 and 10% CaCO₃. The green, hot, fired MOR anderosion test results for shells with and without CaCO₃ stucco coat wereas follows: TABLE 6 Summary Green Data F % CaCO₃ MoR (psi) AFL (lbs) MoE(kpsi) Index Bending (mils)  0 564.40 9.44 268 38.38 6.32  5 566.48 9.11306 33.16 5.78 10 580.84 8.64 305 32.67 6.02 20 577.19 8.64 305 32.676.02 30 559.01 6.71 355 24.90 5.62 Summary hot and fired data Fired %CaCO₃ Hot MoR (psi) MoR (psi) Hot AFL (lbs) Fired AFL (lbs)  0 1326.26932.15 19.28 16.22  5 1127.35 809.54 15.44 11.94 10 923.31 729.07 15.4411.14 20 723.13 558.85 9.75 6.54 30 793.39 496.62 7.81 5.59 Summaryerosion data 2 Hr cooling 24 Hr cooling 48 Hr cooling % CaCO₃ (mils/sec)(mils/sec) (mils/sec) 0 4.89 4.40 4.79 2 4.51 5.13 6.38 4 4.37 10.749.64 6 4.46 10.71 14.71 8 4.34 14.70 25.10 10  4.23 10.58 24.28

While the present invention is described above in connection withpreferred or illustrative embodiments, these embodiments are notintended to be exhaustive or limiting of the invention. Rather, theinvention is intended to cover all alternatives, modifications andequivalents included within its spirit and scope, as defined by theappended claims.

1-3. (canceled)
 4. A method of forming an investment casting shell in a shell preparation sequence and improving the removal of the investment casting shell surrounding a metallic part, wherein the shell preparation sequence comprises deposition of alternating layers of a refractory slurry and a dry refractory stucco onto a pattern, wherein an effective amount of the salt of an alkali or alkaline earth metal is added to at least one layer of the dry refractory stucco during the shell preparation sequence, and wherein the refractory slurry layers are free of added alkali or alkaline earth metal salt during the shell preparation sequence. 5-11. (canceled) 12 The method of claim 4, wherein the salt of the alkali or alkaline earth metal is selected from the group consisting of: calcium carbonate, calcium sulfate, calcium magnesium carbonate, magnesium carbonate, magnesium sulfate, strontium carbonate, and mixtures thereof.
 13. The method of claim 4, including about 1 percent to about 30 percent of the salt of the alkali or alkaline earth metal, based on weight of the shell. 